1. Field of the Invention
This invention relates to circuitry for driving an image sensor, and in particular, to an image sensor driving circuit utilizing the virtual ground of an operational amplifier for driving a plurality of photoelectric elements arranged in the form of an array selectively.
2. Description of the Prior Art
As shown in FIG. 1, in facsimile machines or the like, use is made of a solid-state line sensor, typically in the form a CCD (Charge Coupled Device) structure, which includes a plurality of photoelectric elements S1 through S2,048 arranged in the form of a single line. These photoelectric elements are normally divided into a first plurality of blocks, each containing a predetermined number of photoelectric elements, and the plurality of photoelectric elements are activated block by block for reading an image along the main scanning line, which is defined by the one dimensional array of the plurality of photoelectric elements. In the example shown in FIG. 1, the total of 2,048 photoelectric elements are divided into 128 blocks, and those of the photoelectric elements which belong to the same block are commonly connected together at one end and also connected to the corresponding one of block common electrodes B1, B1'. B2, B2, . . . , B64, and B64'. An activation pulse or driving voltage is applied to the block common electrodes B1 through B64' sequentially one from another so that all of the photoelectric elements belonging to the same block are driven at the same time. In the illustrated example, each block contains sixteen photoelectric elements.
The line image sensor 1 having the total of 2,048 photoelectric elements arranged in the form of a single line and divided into 128 blocks is electrically connected through interconnection lines to an operational amplifier section 2 which includes 32 two-stage operational amplifier circuits. Each of the two-stage operational amplifier circuits includes a first stage operation amplifier OPA1 which has its inverting input terminal connected from the individual end of each of the corresponding photoelectric elements in alternate blocks and has its non-inverting input terminal connected to ground. This first stage operational amplifier OPA1 has its output terminal connected to the inverting input terminal through a feed-back circuit including a parallel connection of a capacitor and a resistor and also to an inverting input terminal of its associated second stage operational amplifier OPA2, which has its non-inverting input terminal connected to ground and its output connected to its inverting input terminal through a feed-back loop containing a resistor and also to device output terminal V.sub.0 through a corresponding one of 32 MOS switches MPX1 through MPX32, which define a multiplexer.
With this structure, the switches MPX1 through MPX32 are turned on sequentially one after another while activating one of the blocks and this operation is repeated while activating another block. As a result, outputs from the plurality of photoelectric elements S1 through S2,048 are obtained in a timed sequence at the device output terminal V.sub.0.
The detailed structure of the two-stage operational amplifier circuit is shown in FIG. 2. As indicated in FIG. 2, since the inverting input terminal of the first stage operational amplifier OPA1 is connected to receive an output signal from one of the photoelectric elements, which are typically comprised of amorphous-Si or the like, there is present a sensor resistance R.sub.p connected in series with the inverting input terminal of the first stage operational amplifier OPA1 and also a parasitic capacitance C.sub.s connected in parallel with the sensor resistance R.sub.p. Accordingly, if a feed-back capacitor C.sub.f is not provided for the first stage operational amplifier OPA1, a differential signal is superimposed in the output voltage V.sub.0 as shown in FIG. 3 and the rising time period for the differential signal can be as long as 20 micro-seconds, which could be a cause of noises. For this reason, it is normally the case to provide the feed-back capacitor C.sub.f to the first stage operational amplifier OPA1 so as to absorb such a differential signal by the feed-back capacitor C.sub.f, thereby allowing to obtain a smoothed output voltage V.sub.0 as shown in FIG. 4. The value of the feed-back capacitor C.sub.f may be determined by the relationship of C.sub.s .multidot.R.sub.p =C.sub.f .multidot.R.sub.f. Typically, it holds that R.sub.p =100 M-ohms and C.sub.s =0.2 pF and thus we obtain C.sub.f .multidot.R.sub.f =20.times.10.sup.6.
On the other hand, the level of input voltage to the analog multiplexer MPX must be set at 2 V or higher in order to prevent its output voltage from being adversely affected by its own noises, and, thus, if the driving voltage V.sub.i to be applied to the block common electrodes B is 12 V, then the resistance of the feed-back resistor R.sub.f of the operational amplifier OPA1 becomes large, i.e., in the order of 20 M-ohms. In this case, therefore, it is necessary to use the feed-back capcacitor C.sub.f having the capacitance in the order of 1 pF; however, it is rather difficult to obtain a trimmer capacitor in the order of 1 pF due to the problem of parasitic capacitance associated with an insulating material, such as PCB, used in mounting an IC chip. Furthermore, the feed-back capacitor C.sub.f is required to be capable of being finely adjusted for compensating the differential signal in consideration of the scatter in the parasitic capacitance C.sub.s among the photoelectric elements.
Under the circumstances, in accordance with the prior art, as shown in FIG. 2, the two-stage operational amplifier circuit including a series-connected operational amplifiers OPA1 and OPA2 is provided and the resistance of the feed-back resistor R.sub.f of the first stage operational amplifier OPA1 is set in the order of 1 M-ohms while using a trimmer capacitor having the capacitance in the vicinity of 20 pF as the feed-back capacitor C.sub.f. The output voltage of the first stage operational amplifier OPA1 is further amplified by the second stage operational amplifier OPA2 thereby allowing to obtain a required level of output voltage. It is to be noted, however, that this prior art approach requires to provide a two-stage operational amplifier circuit, and, thus, it is rather complicated in structure, larger in size and can be a cause in lowering the yield.